Semiconductor device

ABSTRACT

Provided is a semiconductor device including a substrate, a gate structure, a second dielectric layer and a source/drain region. A first dielectric layer is disposed on the substrate, and the first dielectric layer has a trench therein. The gate structure is disposed on the substrate in the trench and includes a work function metal layer and a metal layer. The work function metal layer is disposed in the trench, and includes a TiAl 3  phase metal layer. A height of the work function metal layer disposed on a sidewall of the trench is lower than a height of a top surface of the first dielectric layer. The metal layer fills the trench. The second dielectric layer is disposed between the gate structure and the substrate. The source/drain region is disposed in the substrate at two sides of the gate structure.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of and claims the priority benefit of U.S. application Ser. No. 13/109,599, filed on May 17, 2011, now allowed. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and a method of fabricating the same. More particularly, the present invention relates to a semiconductor device having a metal gate and a method of fabricating the same.

2. Description of Related Art

With the continual miniaturization of semiconductor devices, sizes of gate structures are also gradually reduced. Therefore, the thickness of gate dielectric layers also needs to be reduced to prevent the performance of the devices from being affected. Generally, the material of the gate dielectric layers usually includes silicon oxide. However, the leakage current phenomenon frequently occurs when reducing the thickness of the gate dielectric layers adopting silicon oxide.

In order to reduce the occurrence of leakage current, the conventional method is to replace silicon oxide with high dielectric constant (high k) material for the gate dielectric layers. When applying high dielectric constant material as the gate dielectric layers, the gates adopting polysilicon then react with high dielectric constant material to generate Fermi-level pinning, thereby resulting in an increase in the threshold voltage so as to affect the performance of the device.

In one of the conventional techniques, the metal layer is applied as the gate, that is, the work function metal layer known in the art, to prevent the threshold voltage from increasing and therefore reducing the resistance of the device.

However, conventional semiconductor devices have a difficulty in lowering the band-edge work function, and therefore the semiconductor devices with superior electrical characteristics can not be provided.

SUMMARY OF THE INVENTION

The present invention is directed to a method of fabricating a semiconductor device, and thereby the band-edge work function of the semiconductor device can be effectively lowered.

The present invention is further directed to a semiconductor device with superior electrical characteristics.

The present invention further provides a semiconductor device including a substrate, a gate structure, a second dielectric layer and a source/drain region. A first dielectric layer is disposed on the substrate, and the first dielectric layer has a trench therein. The gate structure is disposed on the substrate in the trench and includes a work function metal layer and a metal layer. The work function metal layer is disposed in the trench, and includes a TiAl₃ phase metal layer. A height of the work function metal layer disposed on a sidewall of the trench is lower than a height of a top surface of the first dielectric layer. The metal layer fills the trench. The second dielectric layer is disposed between the gate structure and the substrate. The source/drain region is disposed in the substrate at two sides of the gate structure.

According to an embodiment of the present invention, in the semiconductor device, the work function metal layer includes a Ti-containing metal layer, the TiAl₃ phase metal layer and an Al metal layer. The TiAl₃ phase metal layer is disposed on the Ti-containing metal layer. The Al metal layer is disposed on the TiAl₃ phase metal layer.

According to an embodiment of the present invention, in the semiconductor device, the work function metal layer includes a Ti-containing metal layer and the TiAl₃ phase metal layer. The TiAl₃ phase metal layer is disposed on the Ti-containing metal layer.

According to an embodiment of the present invention, in the semiconductor device, the work function metal layer includes the TiAl₃ phase metal layer and an Al metal layer. The Al metal layer is disposed on the TiAl₃ phase metal layer.

According to an embodiment of the present invention, in the semiconductor device, a material of the second dielectric layer is a high dielectric constant material, for example.

According to one embodiment of the present invention, the semiconductor device further includes an interfacial layer disposed between the substrate and the second dielectric layer.

According to one embodiment of the present invention, the semiconductor device further includes a barrier layer disposed between the second dielectric layer and the gate structure.

According to one embodiment of the present invention, the semiconductor device further includes a spacer disposed on a sidewall of the gate structure.

According to one embodiment of the present invention, the metal layer is substantially shaped as “T”.

The present invention further provides a semiconductor device including a substrate, a gate structure, a second dielectric layer and a source/drain region. A first dielectric layer is disposed on the substrate, and the first dielectric layer has a trench therein. The gate structure is disposed on the substrate in the trench and includes a work function metal layer and a metal layer. The work function metal layer is disposed in the trench, and includes a TiAl₃ phase metal layer. The metal layer is substantially shaped as “T” and fills the trench. A height of the work function metal layer disposed on a sidewall of the trench is lower than a height of a top surface of the first dielectric layer. The second dielectric layer is disposed between the gate structure and the substrate. The source/drain region is disposed in the substrate at two sides of the gate structure.

According to an embodiment of the present invention, in the semiconductor device, the work function metal layer includes a Ti-containing metal layer, the TiAl₃ phase metal layer and an Al metal layer. The TiAl₃ phase metal layer is disposed on the Ti-containing metal layer. The Al metal layer is disposed on the TiAl₃ phase metal layer.

According to an embodiment of the present invention, in the semiconductor device, the work function metal layer includes a Ti-containing metal layer and the TiAl₃ phase metal layer. The TiAl₃ phase metal layer is disposed on the Ti-containing metal layer.

According to an embodiment of the present invention, in the semiconductor device, the work function metal layer includes the TiAl₃ phase metal layer and an Al metal layer. The Al metal layer is disposed on the TiAl₃ phase metal layer.

According to an embodiment of the present invention, in the semiconductor device, a material of the second dielectric layer is a high dielectric constant material, for example.

According to one embodiment of the present invention, the semiconductor device further includes an interfacial layer disposed between the substrate and the second dielectric layer.

According to one embodiment of the present invention, the semiconductor device further includes a barrier layer disposed between the second dielectric layer and the gate structure, and a spacer disposed on a sidewall of the gate structure.

Based on the above, in the method of fabricating the semiconductor device of the present invention, the work function metal layer is formed by performing the thermal process to anneal the Ti-containing metal layer and the Al layer formed by the physical vapor deposition processes, and therefore the band-edge work function of the semiconductor device can be effectively lowered.

Furthermore, in the semiconductor device of the present invention, the work function metal layer includes the TiAl₃ phase metal layer, and therefore the semiconductor device has superior electrical characteristics.

In order to make the aforementioned and other features and advantages of the invention more comprehensible, embodiments accompanying figures are described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

FIGS. 1A to 1D are schematic cross-sectional views of a fabricating process of a semiconductor device according to an embodiment of the present invention.

FIGS. 2A to 2D are partial sectional views of various work function metal layers.

FIGS. 3A to 3D are schematic cross-sectional views of a fabricating process of a semiconductor device according to another embodiment of the present invention.

FIG. 4 is a schematic cross-sectional view of a semiconductor device according to another embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

FIGS. 1A to 1D are schematic cross-sectional views of a fabricating process of a semiconductor device according to an embodiment of the present invention FIGS. 2A to 2D are partial sectional views of various work function metal layers. It should be noted that the method of fabricating the semiconductor device described in the following adopts a fabricating process of an N-type metal oxide semiconductor (NMOS) device for illustration. People skilled in the art should be able to embody the invention based on the illustration, whereas the scope of the invention is not limited thereto. Moreover, the disposition locations, the manners of formation, and the orders of other components such as gate structures, doped regions, spacers, contact etch stop layers, and the like are all fabricated with techniques known to people skilled in the art based on conventional arts, and are not limited to the descriptions illustrated the following embodiments.

Referring to FIG. 1A, a substrate 100 is provided, wherein a first dielectric layer 102 having a trench 104 therein is formed on the substrate 100, a source/drain region 106 is formed in the substrate 100 at two sides of the trench 104, and a second dielectric layer 108 is formed on the substrate 100 in the trench 104. The substrate 100 is, for example, a semiconductor substrate such as a silicon substrate.

It should be noted that the method of fabricating the first dielectric layer 102, the trench 104 in the first dielectric layer 102, the source/drain region 106, and the second dielectric layer 108 are described for illustration and a scope of the present invention is not limited thereto.

In the present embodiment, the first dielectric layer 102 and the trench 104 disposed therein are fabricated by the following steps. A dummy gate adopting polysilicon (not shown) is formed on the substrate 100, and then the first dielectric layer 102 exposing the dummy gate is formed by a chemical vapor deposition (CVD) process and a chemical mechanical polishing (CMP) process. After that, the dummy gate is removed and the trench 104 is formed in the first dielectric layer 102. The material of the first dielectric layer 102 is, for example, silicon oxide or composite layer of different dielectric materials.

The source/drain region 106 can be formed by performing an ion implantation process after the formation of the dummy gate and before the formation of the first dielectric layer 102. In detail, types, concentrations and depths of the dopants used to form the source/drain region 106 may be adjusted by people skilled in the art based on demands.

Particularly, the second dielectric layer 108 can be formed before the formation of the first dielectric layer 102 or after the formation of the trench 104 in the first dielectric layer 102. The present embodiment takes the second dielectric layer 108 formed before the formation of the first dielectric layer 102 as an example for illustration, and therefore the second dielectric layer 108 is substantially shaped as “-”. The material of the second dielectric layer 108 is, for example, a high dielectric constant (high-k) material. The high dielectric constant material is, for example, a dielectric material with a dielectric constant greater than 4, such as La₂O₃, AlO, ZrO₂, TiO, Ta₂O₅, Y₂O₃, SrTiO₃(STO), BaTiO₃(BTO), BaZrO, HfZrO, HfZrON, HfLaO, HfSiO, HfSiON, LaSiO, AlSiO, HfTaO, HfTiO, (Ba,Sr)TiO₃(BST), Al₂O₃, Si₃N₄, oxynitrides, or a combination thereof. The method of forming the second dielectric layer 108 is an atomic layer deposition (ALD) process, a chemical vapor deposition (CVD) process, a metal-organic CVD (MOCVD) process, a physical vapor deposition (PVD) process, a thermal oxidation process, or a combination thereof.

In addition, in the fabrication of the semiconductor device, an interfacial layer 110, a barrier layer 112, a spacer 114, a contact etch stop layer (CESL) 116 or the combination thereof can be selectively formed, which are all fabricated with techniques known to people skilled in the art based on conventional arts, and therefore they are only described briefly and the scope of the invention is not limited thereto.

In addition, before the formation of the second dielectric layer 108, the interfacial layer 110 may be selectively formed on the substrate 100 in the trench 104. The material of the interfacial layer 110 is silicon oxide, for example. The method of forming the interfacial layer 110 is an atomic layer deposition process, a chemical vapor deposition process, a physical vapor deposition process, a thermal oxidation process, or a combination thereof.

After the second dielectric layer 108 is formed and before the Ti-containing metal layer is subsequently formed, the barrier layer 112 can be selectively formed on the second dielectric layer 108, thereby eliminating Fermi-level pinning generated between the polysilicon dummy gate and the second dielectric layer 108. In addition, the barrier layer 112 may be used as an etch barrier layer when removing the dummy gate. Moreover, the barrier layer 112 can be also used as a diffusion barrier layer to protect the second dielectric layer 108. The material of the barrier layer 112 is TaN or TiN, for example. The method of forming the barrier layer 112 includes an atomic layer deposition process, a chemical vapor deposition process, or a physical vapor deposition process, for example.

Furthermore, after the dummy gate is formed and before the first dielectric layer 102 is formed, the spacer 114 can be selectively fondled on the substrate 100 at two sides of the trench 104. The material of the spacer 114 is silicon oxide, silicon nitride, or silicon oxynitride, for example. In a method of forming the spacer 114, a spacer material layer (not illustrated) is first formed on the substrate 100 by the CVD process, and then a portion of the spacer material layer is removed by an etching back process.

After the spacer 114 is formed and before the first dielectric layer 102 is formed, the contact etch stop layer 116 can be selectively formed on the substrate 100. The material of the contact etch stop layer 116 is, for example, silicon nitride, compressive or tensile silicon nitride, or silicon nitride having an underlying silicon oxide pad layer. The method of forming the contact etch stop layer 116 is a chemical vapor deposition process, for example.

It is mentioned that, when the NMOS device of the embodiment is integrated into the fabrication process of CMOS device, a work function metal layer of TiN is formed in the PMOS device (not shown), and the work function metal layer of TiN is also formed in the NMOS device. Meanwhile, the fabrication process of the NMOS device of the embodiment can further include forming an etching stop layer 117 in the trench 104, and therefore the work function metal layer of TiN can be formed on the etching stop layer 117. As such, the etching stop layer 117 can be used as an etching stop layer in the etching process for removing the work function metal layer of TiN in the NMOS device. The material of the etching stop layer 117 is TaN, for example.

Referring to FIG. 1B, next, a first physical vapor deposition process is performed to form a Ti-containing metal layer 118 on the etching stop layer 117 in the trench 104. The Ti-containing metal layer 118 is a TiAl layer or a Ti layer, for example. In this embodiment, a thickness of the Ti-containing metal layer 118 is from 2 nm to 10 nm, for example. The ratio of Ti to Al in a target used to form the TiAl layer can be from 1:1 to 1:3. The first physical vapor deposition process can be a radio frequency/direct current physical vapor deposition process, thereby reducing the damage to layer caused by plasma.

Then, a second physical vapor deposition process is performed to form an Al layer 120 on the Ti-containing metal layer 118 in the trench 104. In this embodiment, a thickness of the Al layer 120 is from 3 nm to 10 nm, for example. In addition, the second physical vapor deposition process for forming the Al layer 120 can be a cold process. For example, by disposing the substrate 100 on the carrier having cooling function, the substrate 100 can be maintained in a low temperature during the second physical vapor deposition process. The temperature of the cold process is about room temperature or a little bit higher than room temperature, for example.

Referring to 1C, thereafter, a thermal process is performed to anneal the Ti-containing metal layer 118 and the Al layer 120, thereby forming a work function metal layer 122. The temperature of the thermal process is, for example, from 200° C. to 500° C. such as 460° C. The conducting time of the thermal process is from 2 min to 10 min for example. Particularly, the thermal process, the first physical vapor deposition process and the second physical vapor deposition process can be performed in situ, in different reaction chambers of the cluster tool, or by any other processes in which the vacuum is not be broken.

Furthermore, referring to FIGS. 1C and 2A to 2D, in the thermal process, the Ti-containing metal layer 118 and the Al layer 120 react with each other by annealing so as to form a TiAl₃ phase metal layer 124, and therefore the work function metal layer 122 includes the TiAl₃ phase metal layer 124. The structure of the work function metal layer 122 can have various configurations according to extent of reaction between and ratio between thickness of the Ti-containing metal layer 118 and the Al layer 120.

Referring to FIG. 2A, when the Ti-containing metal layer 118 and the Al layer 120 react with each other completely, the work function metal layer 122 can be a single layer of the TiAl₃ phase metal layer 124.

Referring to FIG. 2B, when the Ti-containing metal layer 118 and the Al layer 120 do not react with each other completely and a portion of the Ti-containing metal layer 118 and a portion of the Al layer 120 are remained, the work function metal layer 122 includes the Ti-containing metal layer 118, the TiAl₃ phase metal layer 124 and the Al metal layer 120. In the present embodiment, the TiAl₃ phase metal layer 124 is disposed on the Ti-containing metal layer 118. The Al metal layer 120 is disposed on the TiAl₃ phase metal layer 124.

Referring to FIG. 2C, when the Ti-containing metal layer 118 and the Al layer 120 do not react with each other completely and a portion of the Ti-containing metal layer 118 is remained, the work function metal layer 122 includes the Ti-containing metal layer 118 and the TiAl₃ phase metal layer 124. In the present embodiment, the TiAl₃ phase metal layer 124 is disposed on the Ti-containing metal layer 118.

Referring to FIG. 2D, when the Ti-containing metal layer 118 and the Al layer 120 do not react with each other completely and a portion of the Al layer 120 is remained, the work function metal layer 122 includes TiAl₃ phase metal layer 124 and the Al metal layer 120. In the present embodiment, the Al metal layer 120 is disposed on the TiAl₃ phase metal layer 124.

Referring to FIG. 1C, a metal material layer 126 is then formed on the work function metal layer 122, wherein the metal material layer 126 fills the trench 104. The material of the metal material layer 126 is, for example, a low resistance metal such as Al. The method of forming the metal material layer 126 is a PVD process, for example.

Referring to FIG. 1D, a CMP process is performed to remove the metal material layer 126 outside the trench 104, so as to form a metal layer 128 filling the trench 104. However, the method of forming the metal layer 128 is not limited to the scope of the foresaid method. In addition, the CMP process can remove the work function metal layer 122 and the etching stop layer 117 outside the trench 104 concurrently, and thus only the work function metal layer 122 and the etching stop layer 117 disposed in the trench 104 is remained. After the CMP process is finished, a gate structure 130 including the work function metal layer 122 and the metal layer 128 may be formed in the trench 104.

According to the above embodiments, the work function metal layer 122 is formed by performing the thermal process to anneal the Ti-containing metal layer 118 and the Al layer 120 formed by the physical vapor deposition process, and thus the band-edge work function of the semiconductor device can be effectively lowered.

FIGS. 3A to 3D are schematic cross-sectional views of a fabricating process of a semiconductor device according to another embodiment of the present invention. It is noted that the process illustrated in FIGS. 3A to 3D is performed on the structure of FIG. 1B. The identical numbers in FIGS. 1B and 3A refer to the identical components, and detailed description thereof is omitted herein.

Referring to FIG. 3A, first, a photoresist layer 121 is formed in the trench 104 of the structure of FIG. 1B, and the top surface of the photoresist layer 121 is lower than the top surface of the first dielectric layer 102, for example.

Referring to FIG. 3B, then, a portion of the Al layer 120 and a portion of the Ti-containing metal layer 118 exposed by the photoresist layer 121 are removed, thereby forming an Al layer 120′ and a Ti-containing metal layer 118′ in the trench 104. In addition, the etching stop layer 117 exposed by the photoresist layer 121 can be also removed in this step, thereby forming an etching stop layer 117′ in the trench 104. As such, the height of the Al layer 120′, the Ti-containing metal layer 118′, and the etching stop layer 117′ disposed on the sidewall of the trench 104 is lower than the height of the top surface of the first dielectric layer 102. The method of partially removing the Al layer 120, the Ti-containing metal layer 118, and the etching stop layer 117 is an etching back process, for example.

Next, the photoresist layer 121 is removed. The method of removing the photoresist layer 121 is, for example, a dry photoresist stripping process or a wet photoresist stripping process.

Referring to FIG. 3C, afterwards, a thermal process is performed to anneal the Ti-containing metal layer 118′ and the Al layer 120′, thereby forming a work function metal layer 122′. The forming method, process condition, and the constitution method of the work function metal layer 122′ are similar to those of the work function metal layer 122. Hence, a detailed description thereof is omitted.

Then, a metal material layer 126′ is formed on the work function metal layer 122′, wherein the metal material layer 126′ fills the trench 104. The material of the metal material layer 126′ is, for example, a low resistance metal such as Al. The method of forming the metal material layer 126′ is a PVD process, for example.

Referring to FIG. 3D, a CMP process is performed to remove the metal material layer 126′ outside the trench 104, so as to form a metal layer 128′ filling the trench 104. However, the method of forming the metal layer 128′ is not limited to the scope of the foresaid method. After the CMP process is finished, a gate structure 130′ including the work function metal layer 122′ and the metal layer 128′ may be formed in the trench 104.

According to the above embodiments, the work function metal layer 122′ is formed by performing the thermal process to anneal the Ti-containing metal layer 118′ and the Al layer 120′ formed by the physical vapor deposition process, and thus the band-edge work function of the semiconductor device can be effectively lowered.

Herein, the semiconductor device of the present embodiment is described with reference to FIGS. 1D and 3D.

Referring to FIG. 1D, the semiconductor device 100 includes the substrate 100, the gate structure 130, the second dielectric layer 108 and the source/drain region 106. The first dielectric layer 102 is disposed on the substrate 100, and the first dielectric layer 102 has the trench 104 therein. The gate structure 130 is disposed on the substrate 100 in the trench 104 and includes a work function metal layer 122 and a metal layer 128. The work function metal layer 122 is disposed in the trench 104, and includes a TiAl₃ phase metal layer 124 as shown in FIGS. 2A to 2D. The metal layer 128 fills the trench 104. The second dielectric layer 108 is disposed between the gate structure 130 and the substrate 100. The source/drain region 106 is disposed in the substrate 100 at two sides of the gate structure 130. In addition, the semiconductor device can selectively include the interfacial layer 110, the barrier layer 112, the spacer 114, the contact etch stop layer 116, the etching stop layer 117 or the combination thereof. In detail, the interfacial layer 110 is disposed between the substrate 100 and the second dielectric layer 108. The barrier layer 112 is disposed between the second dielectric layer 108 and the gate structure 130. The spacer 114 is disposed on a sidewall of the gate structure 130. The contact etching stop layer 116 covers the spacer 114 and a portion of the substrate 100. The etching stop layer 117 is disposed in the trench 104, and the work function metal layer 122 is disposed on the etching stop layer 117. It is noted that the material, the forming method and the efficacy of each component in the semiconductor device are described in details in the above embodiments. Hence, a detailed description thereof is omitted.

Moreover, it is mentioned that the difference between the semiconductor devices of FIGS. 3D and 1D is that the height of the work function metal layer 122′ disposed on the sidewall of the trench 104 is lower than the height of the top surface of the first dielectric layer 102, and the metal layer 128′ is substantially shaped as “T”.

Based on the above embodiments, the work function metal layers 122 and 122′ include the TiAl₃ phase metal layer 124, and thus the semiconductor device has superior electrical characteristics.

FIG. 4 is a schematic cross-sectional view of a semiconductor device according to another embodiment of the present invention.

The major difference between the semiconductor devices in FIG. 4 and FIG. 1D relies in that the second dielectric layer 108 and the barrier layer 112 in FIG. 1D are substantially shaped as “-”, and the second dielectric layer 108′ and the barrier layer 112′ in FIG. 4 are substantially shaped as “U”. This difference results from that the second dielectric layer 108 and the barrier layer 112 in FIG. 1D are formed before the formation of the first dielectric layer 102 and the second dielectric layer 108′ and the barrier layer 112′ in FIG. 4 are formed after the formation of the trench 104 in the first dielectric layer 102. The same reference numerals used in FIGS. 3 and 1D are used to refer to the same elements, and thus a detailed description thereof is omitted.

Similarly, the work function metal layer 122 in FIG. 4 includes the TiAl₃ phase metal layer 124, and thus the semiconductor device has superior electrical characteristics.

In addition, reference with the above embodiments, one of the ordinary skill in the art can speculate that the second dielectric layer 108′ and the barrier layer 112′ substantially shaped as “U” in FIG. 4 can be applied to the structure of FIG. 3D, and a detailed description thereof is omitted.

In summary, the above embodiments have at least the following advantages:

1. By the method of fabricating the semiconductor device in the above embodiments, the band-edge work function of the semiconductor device can be effectively lowered.

2. The semiconductor devices in the above embodiments have superior electrical characteristics.

Although the invention has been described with reference to the above embodiments, it will be apparent to one of the ordinary skill in the art that modifications to the described embodiment may be made without departing from the spirit of the invention. Accordingly, the scope of the invention will be defined by the attached claims not by the above detailed descriptions. 

What is claimed is:
 1. A semiconductor device, comprising: a substrate, having a first dielectric layer thereon, wherein the first dielectric layer has a trench therein; a gate structure, disposed on the substrate in the trench and comprising: a work function metal layer, disposed in the trench, and comprising a TiAl₃ phase metal layer, wherein a height of the work function metal layer disposed on a sidewall of the trench is lower than a height of a top surface of the first dielectric layer; and a metal layer filling the trench; a second dielectric layer, disposed between the gate structure and the substrate; and a source/drain region, disposed in the substrate at two sides of the gate structure.
 2. The semiconductor device as claimed in claim 1, wherein the work function metal layer comprises: a Ti-containing metal layer; the TiAl₃ phase metal layer, disposed on the Ti-containing metal layer; and an Al metal layer, disposed on the TiAl₃ phase metal layer.
 3. The semiconductor device as claimed in claim 2, wherein the Ti-containing metal layer comprises a TiAl layer or a Ti layer.
 4. The semiconductor device as claimed in claim 1, wherein the work function metal layer comprises: a Ti-containing metal layer; and the TiAl₃ phase metal layer, disposed on the Ti-containing metal layer.
 5. The semiconductor device as claimed in claim 4, wherein the Ti-containing metal layer comprises a TiAl layer or a Ti layer.
 6. The semiconductor device as claimed in claim 1, wherein the work function metal layer comprises: the TiAl₃ phase metal layer; and an Al metal layer, disposed on the TiAl₃ phase metal layer.
 7. The semiconductor device as claimed in claim 1, wherein the second dielectric layer comprises a high dielectric constant material.
 8. The semiconductor device as claimed in claim 1 further comprising an interfacial layer disposed between the substrate and the second dielectric layer.
 9. The semiconductor device as claimed in claim 1 further comprising a barrier layer disposed between the second dielectric layer and the gate structure.
 10. The semiconductor device as claimed in claim 1 further comprising a spacer disposed on a sidewall of the gate structure.
 11. The semiconductor device as claimed in claim 1, wherein the metal layer is substantially shaped as “T”.
 12. A semiconductor device, comprising: a substrate, having a first dielectric layer thereon, wherein the first dielectric layer has a trench therein; a gate structure, disposed on the substrate in the trench and comprising: a work function metal layer, disposed in the trench, and comprising a TiAl₃ phase metal layer; and a metal layer filling the trench, wherein the metal layer is substantially shaped as “T”; a second dielectric layer, disposed between the gate structure and the substrate; and a source/drain region, disposed in the substrate at two sides of the gate structure.
 13. The semiconductor device as claimed in claim 12, wherein the work function metal layer comprises: a Ti-containing metal layer; the TiAl₃ phase metal layer, disposed on the Ti-containing metal layer; and an Al metal layer, disposed on the TiAl₃ phase metal layer.
 14. The semiconductor device as claimed in claim 13, wherein the Ti-containing metal layer comprises a TiAl layer or a Ti layer.
 15. The semiconductor device as claimed in claim 12, wherein the work function metal layer comprises: a Ti-containing metal layer; and the TiAl₃ phase metal layer, disposed on the Ti-containing metal layer.
 16. The semiconductor device as claimed in claim 15, wherein the Ti-containing metal layer comprises a TiAl layer or a Ti layer.
 17. The semiconductor device as claimed in claim 12, wherein the work function metal layer comprises: the TiAl₃ phase metal layer; and an Al metal layer, disposed on the TiAl₃ phase metal layer.
 18. The semiconductor device as claimed in claim 12, wherein the second dielectric layer comprises a high dielectric constant material.
 19. The semiconductor device as claimed in claim 12 further comprising an interfacial layer disposed between the substrate and the second dielectric layer.
 20. The semiconductor device as claimed in claim 12 further comprising: a barrier layer disposed between the second dielectric layer and the gate structure; and a spacer disposed on a sidewall of the gate structure. 